1. Field of the Invention
This invention relates to a signal processing unit such as an MRE sensor or an image sensor. More particularly, it relates to a sensor signal processing unit which detects magnetism as an electrical signal and digitizes the detected signal.
2. Description of the Related Art
To digitize a sensor output, a method which effects digitization using a trimmed and fixed threshold value matching a sensor output has been generally used in the past.
However, this method cannot digitize with a high level of angle accuracy, when waveforms the peaks and valley of which change in each cycle of waveform or the waveforms have the period of lower frequency superimposed waveform (due, e.g., to eccentricity of a gear).
Therefore, Japanese Unexamined Patent Publication (Kokai) No. 4-77671 proposes a solution. According to this reference, an output signal of a sensor is passed through an amplifier having a large amplification factor and an amplifier having a small amplification factor, and the output signal of the amplifier having a large amplification factor is digitized using a peak hold value of the output signal of the amplifier having a small amplification factor as a threshold value. In other words, two signals having mutually different amplification levels are generated.
The prior art reference described above requires two amplifiers having different amplification factors. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 4-69986 discloses an apparatus for detecting the revolution of a rotary member by using a magnetoresistance element (MRE) (for example a ferromagnetic thin film resistor). When a gear-like rotary member rotates, the direction of a magnetic flux passing through the MRE changes, so that the output of the MRE also changes. The reference detects the revolution of the rotary member by digitizing the output change and effecting signal processing.
To convert the wave-like output from the MRE a digital output, or in other words, to digitize the output, the reference described above employs the following method to determine a threshold value. Namely, the method sets in advance the threshold values (an upper limit value S1 and a lower threshold value S2) of a binary pulse-forming comparator as digitization means, and always corrects the output of the MRE in such a manner that the output of the MRE crosses the threshold value, that is, in such a manner that digitization and detection can be carried out even when the output of the MRE changes.
To correct the output, a comparator for setting the upper limit and another comparator for setting the lower limit are provided, and when the output of the MRE exceeds the upper or lower limit value, a count value of an up/down counter is counted down or up using an oscillator to change the count value. This count value is used as an offset input, and offset adjustment is carried out so that the output of the MRE exists between the upper limit value and the lower limit value.
However, the output of the MRE includes an offset voltage and changes with the passage of time. Therefore, output correction must be always carried out so that the output of the MRE exists within the range of the set values as described above. In other words, the output correction circuit must be always kept operating. This means that the oscillator of the output correction circuit is always operating and an oscillation spike noise always occurs. If this spike noise occurs at the same time as the switching operation of the binary pulse forming comparator for digitizing the output of the MRE, the timing of the switching deviates somewhat, and this deviation results in an error in angle accuracy. This term "angle accuracy" means the level of reproducibility which represents at which level of the output waveform of the MRE sensor the digitization output is switched from 1 to 0 and vice versa. Accordingly, if the spike noise occurs, a very small deviation of the edge occurs when the binary pulse-forming comparator effects its comparison operation, and this deviation results in an error in angle accuracy. In the case of a crank angle/cam angle sensor used for controlling an engine of a vehicle, for example, absolute angle accuracy of .+-.0.1.degree. and repeatability of 0.02.degree. are requisites. Accordingly, external noises such as switching noise must also be cut off as much as possible.
It is therefore an object of the present invention to provide a sensor signal processing unit capable of simplifying a circuit construction in view of the problems described above. It is another object of the present invention to provide a sensor signal processing circuit capable of stopping the oscillation operation during normal operation after output correction.